The health risk associated with tobacco use in people with severe mental illness is much higher than the general population. Among them, patients with schizophrenia are the population with perhaps the highest risk for nicotine addition. Current conceptualizations of the possible etiologies of the severe schizophrenia-smoking comorbidity include self-medication for neurocognition deficits, overcoming antipsychotic medication side effects, and shared nicotinic molecular pathways. However, the underlying brain circuitry for the comorbidity is unknown. Identifying brain comorbidity circuitry is critical for developing valid biomarkers for clinical therapeutic development, individualizing treatment selection and outcome prediction. Recent data suggest that the cingulate-ventral striatum circuit appears to be one of the key pathways associated with nicotine addiction. These same areas are also among the regions most commonly implicated in schizophrenia, and additional preliminary studies suggest that the same circuit is impaired in schizophrenia. Therefore, we hypothesize that abnormal cingulate-ventral striatum circuit is a key path for schizophrenia/nicotine addiction comorbidity. We propose to test the hypotheses that the cingulate-ventral striatum circuit is abnormal in nicotine addiction and in schizophrenia, and that schizophrenia pathology disrupts this circuit and predisposes patients to more severe nicotine addiction, leading to the severe nicotine addiction/schizophrenia comorbidity. We will also examine the clinical validities of the circuit in prediction of long-term smoking behavioral change, in genetics, and in its relationships to putative addiction mechanisms. Identifying the key brain circuits associated with smoking in this high risk population will provide concrete biomarkers for new therapeutic development, and ultimately reducing the smoking related health burden in schizophrenia patients. In addition, as public health efforts have reduced smoking, those who still smoke in the population may be more dependent and more treatment-resistant. An effort targeting the population most addicted to smoking may also yield novel perspectives to treat nicotine addiction in the general population.